Front and rear suspensions have improved the performance and comfort of mountain bicycles. Over rough terrain the suspension system can improve traction and handling by keeping the wheels on the ground. A rider can more easily maintain control at higher speeds and with less effort when the suspension absorbs some of the shock encountered when riding. Ideally the suspension should react well to both (1) low amplitude, high frequency bumps and (2) high amplitude, low frequency bumps. However, these can be competing requirements for the damping systems in conventional shock absorbers.
Higher rebound damping is desirable for high amplitude, low frequency bumps than for low amplitude, high frequency bumps. With high frequency, low amplitude bumps, such as may be encountered on a washboard gravel fireroad, minimal damping may be preferable so the spring can quickly recover from a minor impact before the next is encountered. However, with a large bump (such as the size of a curb) increased rebound damping aids the rider by keeping the bike from forcefully springing back too quickly, causing loss of traction and control on the rebound. Compression damping will also stop the bike from bottoming out with large bumps and make for a smoother absorption of the bumps.
Some current shock absorbers that include springs and dampeners allow the rider to adjust rebound and/or compression damping before a ride. Other air shock absorbers include an on/off switch to disable the shock absorber all together. However, such preadjustment is at best a compromise; the rider must select better damping in one scenario at the expense of the other. A typical off-road mountain bike ride will include small bumps, medium, and large bumps, as well as possibly jumps, drop-offs, and tight descending to ascending transitions. If the rider significantly reduces the damping to ride smoothly over high frequency, low amplitude bumps then the bike may lose traction and control when a large bump is encountered or may "bottom out" the shock absorber. If the rider increases the damping force of the shock absorber, then the system will not recover fast enough to quickly absorb high frequency bumps, the rider will be rattled, and the bike will lose traction.
Another limitation of current shock absorbers is evidenced by rider-induced bobbing: suspension movement caused by rider movement during pedaling. Related to this is pedal-induced suspension action: the cyclic forces on the chain pulling the rear swingarm up or down relative to the frame. If the damping in the shock absorber is greater, these influences will not be felt as much by the rider. However, a stiff suspension, especially at the beginning of the stroke of the shock absorber, can decrease the ability of the suspension to absorb small bumps well.
Attempts to overcome the current limitations in suspension systems have focused on swingarm linkages and pivot arrangements. At a significant cost, some amelioration of rider- or pedal-induced suspension action has resulted, but much less progress has been made on the dilemma of large and small bump absorption.